Attenuation of Photoelectron Emission by a Single Organic Layer

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler–DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

the experimental geometry. In both systems, a similar deposition rate in the range between 0.1 ML/min and 0.3 ML/min was used and the sample was kept at room temperature (RT).
Since there was no active temperature control, local heating due to the incident light or the electron beam cannot be excluded. We assume a constant sticking coecient during deposition to convert the deposition time to a coverage scale. This assumption is supported by the observation of PEEM images, which are homogeneous over the eld of view, after equal times in each case see rst column of Fig. S2. We conclude from these homogeneous PEEM images that the rst, second and third layers were completelly lled. Following the analysis presented in Ref. 3, the coverage is expressed as an equivalent of the third layer.

Anderson method
With the Anderson method, relative changes of the work function ∆W are measured using an electron beam, which is directed normally towards the Ag(100) surface as described in Changes in R are evident from the increase of the maximum current (I max ) during CoPc deposition. The total current from the electron gun (I tot ) is measured by applying a voltage of +180 V to the sample. The electron reectivity R is calculated according to: The evolution of the electron reectivity R upon CoPc deposition is shown in Fig. S1(c).
The minimum of R is associated with the completion of the rst monolayer. The maximum current stays constant during further deposition of CoPc, i.e., for coverages Θ ≥ 1 ML.
This indicates the negligible inuence of the thickness of the organic layer on the maximum current extracted from I(V ) curves.
To reliably determine the shift between the I(V )-characteristics for increasing coverage Θ, one has to compensate for the eect of the changing low-energy electron reectivity R.
This is done by normalizing the current I by the (local) maximum current I max : The so normalized I(V )-curves are shown in Fig. S1(b). In the next step, the voltages corresponding to a xed level of I (here 20 %) are determined. The dierence to the value for the initial curve (bare Ag(100) surface) then gives the work function shift ∆W .

PEEM measurements
The experiments were carried out with a commercial PEEM from Focus with integrated sample stage. For the photoelectron excitation, a high-pressure Hg lamp was used, which has a strong emission line at λ = 253 nm (corresponding to hν = 4.9 eV). 4 The molecular source and the lamp were mounted at an angle of 65 • with respect to the surface normal.
A commercial evaporator (ventiotec OVD3) with quartz crucibles was used to deposit the

Fowler-DuBridge
The conversion between work function changes ∆W measured with the Anderson method and those extracted from the mean electron yield MEY is based on the Fowler-DuBridge theory. 68 A central parameter for this conversion is the photon energy (here hν = 4.9 eV).
Since the Anderson method does not provide the absolute value of the work function of the bare substrate W 0 or the thin lm, the initial work function is an additional parameter, which has to be determined independently. In Fig. 2    The best match between the Anderson data and the PEEM ones is found by eye for an initial work function of 4.64 eV.